1. Field of the Invention
The present invention relates to a branching filter including filters having piezoelectric thin film resonators, and the branching filter is preferably for use in a communication device, for example.
2. Description of the Related Art
In recent years, piezoelectric thin film filters using elastic bulk waves have been developed.
Such piezoelectric thin film filters are compact in size, are light weight, and have excellent vibration resistance and impact resistance. In addition, the piezoelectric thin film filters have small variation in products and high reliability, and can provide non-adjusting circuits. Therefore, the mounting process can be automated and simplified. Furthermore, even when the frequency is increased, the piezoelectric thin film filters can be easily produced. Thus, the piezoelectric thin film filters have superior characteristics.
A branching filter (duplexer) including such piezoelectric thin film filters has been proposed.
For example, Japanese Unexamined Patent Application Publication No. 2001-24476 discloses a branching filter including piezoelectric thin film filters in which the piezoelectric thin film resonators are arranged in a ladder configuration.
The piezoelectric thin film resonators used in the branching filter disclosed in Japanese Unexamined Patent Application Publication No. 2001-24476 define a transmitting filter and a receiving filter. In both filters, the electrodes are composed of Mo and piezoelectric thin films are composed of AlN.
However, the required characteristics are different between the transmitting filter and the receiving filter in the branching filter.
In other words, piezoelectric thin film resonators having the same structure are optimized only in either the transmitting filter or the receiving filter.
According to the Japanese Unexamined Patent Application Publication No. 2001-24476, the transmitting filter and the receiving filter have the same structure. Therefore, a branching filter having optimum characteristics in both transmitting and receiving cannot be achieved.